The present invention relates to heat curable silicone rubber compositions. More particularly, this invention relates to heat curable silicone rubber compositions having improved tear strength, oil resistance and compression set properties.
Heat curable silicone rubber compositions are known in the art. Such compositions are known for their ability to resist change at elevated temperature and exposure to adverse weather conditions over extended periods of time. In particular applications, however, such as automobile seals and gaskets, materials are needed which have not only the outstanding properties of organopolysiloxane elastomers but also have a satisfactory degree of toughness. Toughness can be measured by tear strength (pounds per inch, i.e. "pi"), which is the ability of the rubber to withstand continual abrasion without breakdown in mechanical structure. In addition to toughness, elastomeric materials for use in automobiles must have a satisfactory degree of resiliency or ability to recover shape after deformation especially by compressive forces.
Efforts have been undertaken in the past to produce heat curable silicone elastomer compositions having increased tear strength and reduced compression set properties.
For example, U.S. Pat. No. 4,539,357 (Bobear '357) discloses a heat-curable silicone composition comprising a vinyl-terminated linear diorganopolysiloxane gum having a viscosity in the range of 1.times.10.sup.6 to 20.times.10.sup.7 centipoise at 25.degree. C., a vinyl-containing diorganopolysiloxane gum having a viscosity varying from 1.times.10.sup.6 to 20.times.10.sup.7 centipoise at 25.degree. C., a filler, a hydride-containing polysiloxane, and an organic peroxide or organic hydroperoxide curing agent. The composition may further comprise a vinyl-containing organopolysiloxane resin copolymer. It is stated in Bobear '357 at column 6, lines 22-29 that there should be no vinyl-containing fluid in the composition since it has been found that compositions containing vinyl-containing fluids of a viscosity of 500,000 centipoise or less result in elastomers with good tear strength initially, but after the composition has been subjected to post-cure, its tear strength properties degrade dramtically. After post-cure, the Bobear '357 composition has a tear strength of above 200 pi.
U.S. Pat. No. 3,652,475 to Wada et al. (Wada) discloses heat curable elastomeric silicone compositions reportedly having high tear strength, excellent compression set and resiliency, comprising two vinyl-unsaturated polydiorganosiloxanes each having a degree of polymerization of at least 3000; a vinyl-unsaturated polydiorganosiloxane having a degree of polymerization of from 10 to 1000; a silica filler; and an organic peroxide catalyst. The samples prepared in the Wada examples had tear strength values of about 40 kg/cm, i.e., about 225 pi.
U.S. Pat. No. 3,671,480 to Wada et al. (Wada) discloses a heat curable elastomeric silicone composition having improved tear strength and superior compression set, comprising a mixture of two vinyl-unsaturated polydiorganosiloxanes, one of which appears to have a high molecular weight and the other of which appears to have a low molecular weight; a polyorganohydrogensiloxane; silica filler; and a platinum compound. The examples in Wada report tear strength values of about 50 kg/cm, i.e., about 280 pi. Compression set values are not given.
U.S. Pat. No. 4,061,609 to Bobear discloses a platinum catalyzed silicone rubber composition comprising a vinyl-containing polysiloxane or a blend of such polysiloxanes; platinum; and a hydrogen-containing polysiloxane. Preferably, the vinyl-containing polysiloxane has a viscosity in the range of 1000 to 300,000,000. A low viscosity vinyl-containing polysiloxane may be added to the basic composition as a reinforcing agent to give the final composition good physical strength. Claim 4 in Bobear recites a first polysiloxane having a viscosity of 1,000,000 to 200,000,000 centipoise at 25.degree. C. and a second polysiloxane having a viscosity of 50,000 to 500,000 centipoise at 25.degree. C. The Bobear patent is directed to improving the work life of silicone rubber compositions, rather than tear strength. In the examples, tear strength values of greater than 200 pi were obtained, but the compositions having these values contained two high viscosity vinylpolysiloxanes rather than one high viscosity vinylpolysiloxane and one low viscosity vinylpolysiloxane.
U.S. Pat. No. 3,660,345 to Bobear discloses organopolysiloxane compositions which are convertible to elastomers having high tear strength and resiliency, comprising a two component blend of vinyl-containing organopolysiloxanes; silica filler, and a process aid which can be, for example, a methoxy-terminated polysiloxane, a silanol-terminated polydimethylsiloxane, or hexamethyldisilazane. A peroxide is used as the catalyst. The viscosity of one of the two vinyl-containing organopolysiloxanes can be in the range of 1 to 1 billion centipoise at 25.degree. C., while the viscosity of the other can be between 100,000 to 2,000,000,000 centipoise at 25.degree. C. The examples in Bobear report tear strength values of greater than 200 pi.
U.S. Pat. No. 3,696,068 to Creamer discloses heat-curable silicone gumstock compositions having high die "B" tear strength as well as low compression set values, comprising a vinyl-containing diorganopolysiloxane gum and a linear vinyl-containing fluid having a viscosity of from 10 to 150,000 centistokes at 25.degree. C. The crosslinkable vinyl siloxane gum has a viscosity in excess of 200,000 centistokes at 25.degree. C. The Creamer composition is cured by a peroxide catalyst. Some of the samples prepared in Creamer had tear strength values in excess of 150 pi. Tear strength after post cure was not measured in any of the examples, which is significant since, as pointed out previously herein, it has been found that, in compositions containing low viscosity vinyl-containing fluids, tear strength drops dramatically after post cure.
U.S. Pat. No. 3,884,866 to Jeram et al. (Jeram) discloses a high strength organopolysiloxane composition suited for low pressure injection molding, comprising (A) a first component containing (i) a vinyl-containing high viscosity organopolysiloxane having a viscosity of 5000 to 1,000,000 centipoise at 25.degree. C.; and (ii) a low viscosity vinyl-containing organopolysiloxane having a viscosity of 50 to 5000 centipoise at 25.degree. C.; (iii) a filler; and (iv) a platinum catalyst; and (B) a hydrogen silicone composition. The highest tear strength reported in the examples was 250 pi.
Although heat curable silicone rubber compositions having good tear strength and low compression set properties are known in the art, it is continually desirable to provide heat curable silicone rubber compositions having improved tear strength and compression set properties.
Therefore, it is an object of the present invention to provide a heat curable silicone rubber composition with higher tear strength and lower compression set than the silicone rubber compositions described above.
It is a further object of the present invention to provide an oil resistant heat curable silicone rubber composition having high tensile strength and elongation properties in addition to high tear strength and reduced compression set.
These objects are achieved in the present invention.